(1) Field of the Invention
This invention relates to a passive seat belt system adapted to be worn without bothering an occupant of an automotive vehicle for protecting the occupant from collision, and specifically to a drive mechanism for a silent, rattle-free passive seat belt.
(2) Description of the Related Art
Among the various drive mechanisms for passive seat belt systems is the type where a webbing is fastened to a slider guided by a guide rail, the slider being driven by a motor so that the webbing can move between the occupant restraint and occupant release positions. In drive mechanisms of this type, clearance exists between the slider and each face of the associated guide rail. This clearance is essential to allow the slider to glide freely.
However, because of practical machining and production tolerance limitations and the like, this clearance is not precisely uniform, not only in a single rail but also among production lots.
Too great a clearance makes it possible for the slider to rattle inside the guide rail, as sliding resistance and shoulder webbing tension vary. Consequently, the slider is repeatedly caught up and stopped --snagged--inside the guide rail, allowing so-called "chattering vibrations" and unpleasant noise and vibrations which may irritate the occupant.
The causes of such chattering vibrations are complex. The degree of the clearance between the slider and guide rail, the degree of the sliding resistance between the slider and guide rail, variations in the magnitude of the force by which the slider is pushed by the drive member, etc., are all believed to have a bearing. Chattering vibrations are also believed to occur when these causes meet certain conditions.
Based on a simplified model, the mechanism of occurrence of chattering vibrations may be described as follows.
In a seat belt mechanism of conventional type, where the slider is dragged along a glide channel by an associated drive member, undue clearance may allow the slider to make contact with the guide rail on only two points. In this case, the drive force twists the slider sufficiently for it to catch instead of advancing smoothly. As the force increases further, the slider is pulled free of its temporary impediment and advances a little. However, it then catches and stops again. It is believed that repetition of the above results in the vibrations.
If resistance is sufficiently small, the slider will move smoothly without the above problem. However, it is impossible to reduce sliding resistance because the coefficient of dynamic friction has at least a certain level of 0.2 or so and cannot be reduced to where such chattering vibrations can assuredly be eliminated. If the clearance is extremely wide, the slider may twist even to a diagonal position, causing it to snag in the guide rail. This is unlikely however in view of the probability of reasonable machining accuracy. In practice therefore the avoidance of chattering vibrations in such a design is not feasible because the drive force and webbing tension vary inter alia under conditions which also include sliding resistance of at least a certain level and clearances in a certain range.
Under these conditions, greater clearances encourage chattering vibrations whereas reduced clearances can inhibit or reduce them, even under quite adverse conditions. This indicates the importance of minimizing clearance, yet not so much that the slider may jam in the rail. The solution is to provide an automatic adjustment mechanism which always controls the degree of clearance to a suitable level.